Propylene glycol and polypropylene glycols are important intermediates, for example, for producing polyester and epoxy resins and also polyurethane forms. Their industrial preparation has hitherto mostly been carried out using a 3-stage process.
In this process, propylene oxide is reacted with a stoichiometric excess of water at from 180 to 220° C. and a pressure of from 15 to 25 bar in a first stage. Tube reactors connected in series are generally used for this reaction. The propylene glycols obtained are then present in the form of a mixture comprising propylene glycol, also known as monopropylene glycol or 1,2-propylene glycol, dipropylene glycol and tripropylene glycol in a weight ratio of about 100:10:1. This mixture is transferred, in a second stage, to a dewatering apparatus in which the excess water is separated off in a multistage evaporation and is returned to the reaction. Finally, in a third stage, the propylene glycols are distilled to give pure products. According to the prior art, this is carried out using three columns which are connected in series and each have a side offtake. In the first column, the propylene glycol is obtained from the side offtake and the bottoms are passed to the second column from whose side offtake the dipropylene glycol is obtained. The mixture from the bottom of this second column is passed to the third column from whose side offtake the tripropylene glycol can finally be obtained (Ullmann's Encyclopadie der technischen Chemie, 4th Edition, Volume 19, pages 425-432).
The process described requires a high outlay in terms of apparatus and energy, particularly in the process step in which the three propylene glycols mentioned are separated by distillation, since three columns are used. Furthermore, it requires the use of pure propylene oxide as starting material.